It is often observed that monomers that do not readily homopolymerize are able to undergo copolymerization reactions with appropriate co-monomers. The most typical situation occurs when a strong electron donating monomer is mixed with a strong electron accepting monomer from which a regular alternating copolymer results after free radical initiation. Maleic anhydride is a widely used example of a strong electron accepting monomer. Styrene and vinyl ethers are typical examples of electron donating monomers. Systems, such as maleic anhydride—styrene, are known to form charge transfer complexes, which tends to place the monomers in alternating sequence prior to initiation. The application of the free radical initiator “ties” the ordered monomers together to form an alternating copolymer (Cowie, Alternating Copolymers, Plenum, New York (1985)).
U.S. Pat. Nos. 2,378,629 to Hanford and U.S. Pat. No. 4,151,336 to Sackmann et al. disclose that even when a moderately electron donating monomer, such as diisobutylene, is copolymerized with a strong electron acceptor monomer, such as maleic anhydride, an alternating copolymer results.
When a moderately electron donating monomer, such as isobutylene, is copolymerized with a moderately electron accepting monomer, such as an acrylic ester, poor incorporation of the electron donating monomer results. For example, free radical copolymerization of isobutylene (IB) and acrylic monomers has resulted in copolymers that contain at most 20-30% of IB and have low molecular weights because of degradative chain transfer of IB. Examples of such copolymerizations of IB are disclosed in U.S. Pat. No. 2,411,599 to Sparks et al. and U.S. Pat. No. 2,531,196 to Brubaker et al.
The ability to make copolymers of acrylic monomers and IB type monomers is desired in the art. For example, many patents express the potential for using IB-containing polymers in coating compositions. U.S. Pat. No. 6,114,489 to Vicari et al. discloses a coating composition that includes a functional acrylic resin binder; a co-reactant capable of reacting with the functionality of the acrylic binder; a degasser; and a hyperbranched polyester flow and leveling agent. IB is suggested as a potential co-monomer for use in the acrylic binder as part of a long list of monomers. U.S. Pat. No. 5,552,487 to Clark et al. discloses powder coating compositions that include a copolymer having a reactive functionality and a suitable crosslinking agent capable of reaction with the reactive functionality of the copolymer. The copolymer is made by copolymerizing functional monomers with other monomers, isobutylene being one among many listed as potential co-monomers. Although only two are referenced herein, of the many patents that express the possibility of using isobutylene-type co-monomers, none actually shows or discloses a working example of such a copolymer.
The fact that few examples of isobutylene-type monomer-containing copolymers are found is due to the generally non-reactive nature of isobutylene with acrylic and methacrylic monomers. Reactivity ratios for monomers can be calculated using the Alfrey—Price Q-e values (Robert Z. Greenley, Polymer Handbook, 4th Ed., Brandrup, Immergut and Gulke, editors, Wiley & Sons, New York, N.Y., pp. 309-319 (1999)). The calculations may be carried out using Formulas I and II:r1=(Q1/Q2)exp{−e1(e1−e2)}  Ir2=(Q2/Q1)exp{−e2(e2−e1)}  IIwhere r1 and r2 are the respective reactivity ratios of monomers 1 and 2, and Q1 and Q2 and e1 and e2 are the respective reactivity and polarity values for the respective monomers (Odian, Principals of Polymerization, 3rd Ed., Wiley-Interscience, New York, N.Y., Chapter 6, pp. 452-467 and 489-491 (1991)). Table 1 shows the calculated reactivity ratios of selected monomers with isobutylene:
TABLE 1Monomerr1 (isobutylene)r2Methyl acrylate0.1013.67Glycidyl methacrylate0.0834.17Methacrylic acid0.0939.71
As one skilled in the art of polymer chemistry can appreciate, when r1 is near zero and r2 has a value of 10 or more, monomer 2 is reactive toward both monomers and monomer 1 is reactive toward neither monomer. In other words, it is extremely difficult to prepare copolymers having significant amounts of both monomers. It is not surprising then that few examples can be found of coating compositions that include isobutylene-type monomer-containing copolymers, because the monomers do not tend to copolymerize.
A few examples of acrylic ester or acrylonitrile copolymers made by copolymerizing with monomers such as propylene, isobutylene, and styrene have been accomplished in the presence of Lewis acids, such as alkylaluminum halides, to give 1:1 alternating copolymers. The alternating copolymers were obtained when the concentration ratio of the Lewis acids to the acrylic esters was 0.9 and the concentration of IB was greater than the concentration of the acrylic esters (Hirooka et al., J. Polym. Sci. Polym. Chem., 11, 1281 (1973)). The metal halides vary the reactivity of the monomers by complexing with them. The electron donor monomer-electron acceptor monomer-metal halide complex leads to alternating copolymers (Mashita et al., Polymer, Vol. 36, No. 15, pp. 2973-2982, (1995)).
Copolymers of IB and methyl acrylate (MA) have also been obtained by using ethyl aluminum sesquichloride and 2-methyl pentanoyl peroxide as an initiating system. The resulting copolymer had an alternating structure, with either low (Kuntz et al., J. Polym. Sci. Polym. Chem., 16, 1747 (1978)) or high isotacticity in the presence of EtAlCl2 (10 molar % relative to MA). (Florjanczyk et al., Makromol. Chem., 183, 1081 (1982)).
Another method for making IB copolymers with acrylic esters involved alkyl boron halide, which was found to be much more active than alkyl aluminum halides in forming alternating copolymers. The resulting copolymer was an elastomer of high tensile strength and high thermal decomposition temperature with good oil resistance, especially at elevated temperatures (Mashita et al., Polymer, 36, 2983 (1995)).
U.S. Pat. No. 5,807,937 to Matyjaszewski et al. discloses a method of making alternating copolymers of isobutylene and methyl acrylate using atom transfer radical polymerization (ATRP) processes. The method requires the use of a suitable ATRP initiator, such as 1-phenylethyl bromide, and suitable transition metal salts, such as CuBr, with a ligand, such as 2,2′-bipyridyl, to perform the complex redox initiation and propagation steps of the polymerization process.
Copolymers containing relatively high amounts (≧30 mole %) of IB and acrylic esters have only been attained by free radical polymerization when Lewis acids or ATRP initiation systems have been employed. The polymer that results from such processes requires expensive and time consuming clean-up to remove the transition metal salt and/or Lewis acid residues in order to make the polymer commercially useful.
Copolymer compositions that contain Lewis acids and/or transition metals intermingled with the copolymer can have a number of drawbacks when used commercially. Some Lewis acids and transition metals are toxic and have adverse environmental effects if they are leached from the copolymer and enter the environment. In coating applications, the Lewis acids and transition metals may lead to poor stability when exposed to UV light or simply cause the coating to discolor. In other applications, the Lewis acids and transition metals may react with other ingredients in a formulation resulting in undesired properties.
One method of overcoming the problems described above is disclosed in co-pending U.S. patent application Ser. No. 10/077,559, which is directed to a method of making a copolymer containing isobutylene-type co-monomers. The method includes the steps of (a) providing a monomer composition that includes an isobutylene-type monomer; (b) mixing the monomer composition in (a) with an ethylenically unsaturated monomer composition that includes one or more acrylic monomers, and (c) polymerizing the mixture resulting from step (b) in the presence of a free radical polymerization initiator. The polymerization is carried out in the substantial absence of Lewis acids and/or transition metals. The isobutylene-type monomer is present at a molar excess of at least 10 mole % based on the molar concentration of acrylic monomers.
However, the significant excess of isobutylene monomers utilized in the above-described process, results in significant levels of unreacted monomers being present with the copolymer, which must be removed therefrom prior to using the copolymer commercially. Removal of the unreacted monomer can be time consuming and expensive. Furthermore, even when the unreacted monomers can be removed from the copolymer, the ability to recycle the monomers is limited as initiator by-products and residues often contaminate the recovered monomers. The latter situation results in waste generation if the monomers are subsequently discarded and added expense if additional separation techniques are used to isolate pure monomers that can be recycled into the polymerization process.
Therefore, there is a clear and present need for a method for making copolymers containing olefinic monomers, and in particular, isobutylene-type monomers, that does not rely on Lewis acids and transition metals to obtain an alternating copolymer and which minimizes or eliminates unreacted monomer contamination in the final copolymer.